This project will develop a transparent electrode array that can be used to measure extracellular voltages and transmembrane voltage-sensitive dye fluorescence simultaneously from the same locations in the heart. Electrodes will be fabricated using microphotolithographic techniques and thin film deposition of tin oxide. The first electrode arrays will be fabricated and used to quantify the electrical and optical properties of the tin oxide film that are important for the electrical and optical measurements. Then electrode arrays will be fabricated as prototypes for the simultaneous electrical and optical measurements. These arrays will be used to demonstrate that the electrodes can simultaneously measure cardiac electrical signals and optical signals from the same locations in arterial-perfused rabbit hearts. The arrays will be used to obtain preliminary data that will allow Dr. S. Knisley to submit a new R01 grant application, which will have the objective of validating the commonly-used extracellular recording method at the sites of arrhythmic conditions including block, wavefront collision, reentry and multiple wavelets of fibrillation. This will improve the interpretation of extracellular signals, which in turn will allow better determination of sites of origins of arrhythmias for ablation procedures in patients and allow better determination of mechanisms of arrhythmias from extracellular recordings. But first it is necessary to determine whether it is possible to fabricate an electrode array to measure extracellular voltages and transmembrane voltage-sensitive dye fluorescence simultaneously from the same locations in the heart. This project has significant risk since they don't know the electrochemical properties of the interface between tin oxide and the heart tissue, they don't know the optical properties of the tin oxide and they don't know if one can put together an array structure that contains all of the necessary electrical and optical properties.